1. Field of the Invention
This invention is concerned with an improved catalytic process for the demetalation and desulfurization of petroleum oils, preferably those residual fractions that have undesirably high metals and/or sulfur content. More particularly, the invention involves two catalysts with distinctly different pore sizes, arranged in a dual catalyst system that is especially effective for the demetalation and desulfurization of petroleum oils. Both catalysts are exemplified by the cobalt-molybdenum on alumina type.
2. Description of the Prior Art
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals and sulfur content. This comes about because practically all of the metals present in the original crude remain in the residual fraction, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalytic processing such as catalytic cracking and hydrocracking. This is so because the metal contaminants deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolitic operation known as coking. In this operation the residum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800 to 1100.degree. F temperature and a pressure of one to ten atmospheres. The economic value of the coke by-product is determined by its quality, especially its sulfur and metals content. Excessively high levels of these contaminants makes the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 p.p.m. (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high valued metallurgical, electrical, and mechanical applications.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content in many cases is unacceptable for ecological reasons.
At present, catalytic cracking is generally done utilizing hydrocarbon chargestocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree. to 1500.degree. F, a pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 1000 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor". This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m = Fe + V + 10 (Ni + Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25, or even 2.5 to 50, may be used to blend with or as all of the feedstock to a catalytic cracker, since chargestocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance with the newer fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions (having a metals factor of about 2.5 to 50) suitable for cracking chargestocks.
Metals and sulfur contaminants present similar problems with regard to hydrocracking operations which are typically carried out on chargestocks even lighter than those charged to a cracking unit. Hydrocracking catalyst is so sensitive to metals poisoning that a preliminary or first stage is often utilized for trace metals removal. Typical hydrocracking reactor conditions consist of a temperature of 400.degree. to 1,000.degree. F and a pressure of 100 to 3,500 p.s.i.g.
It is evident that there is considerable need for an efficient method to reduce the metals and/or sulfur content of petroleum oils, and particularly of residual fractions of these oils. While the technology to accomplish this for distillate fractions has been advanced considerably, attempts to apply this technology to residual fractions generally fail due to very rapid deactivation of the catalyst, presumably by metals contaminants.
U.S. Pat. No. 3,730,879 issued May 1, 1973 discloses a two-bed catalytic process for the hydrodesulfurization of crude oil or a reduced fraction, in which at least 50 percent of the total pore volume of the first-bed catalyst consists of pores in the 100-200 Angstrom diameter range.
U.S. Pat. No. 3,830,720 issued Aug. 20, 1974 discloses a two-bed catalytic process for hydrocracking and hydrodesulfurizing residual oils, in which a small pore catalyst is disposed upstream of a large-pore catalyst.
U.S. Pat. No. 3,876,523 issued Apr. 8, 1975 describes a novel catalyst and a process for catalytically demetalizing and desulfurizing hydrocarbon oils comprising residual fractions. This entire specification is incorporated herein by reference. The process described therein utilizes a catalyst comprising a hydrogenation component, such as cobalt and molybdenum oxides, composited on an alumina at least a portion of which is in the delta and/or theta phase, with at least 60% of the pore volume of the catalyst in pores having a diameter of 100 Angstroms to 200 Angstroms, also having at least about 5% of the pore volume contributed by pores having a diameter greater than 500 Angstroms and having other characteristics as hereinafter described. As will be shown, although this catalyst is highly effective for demetalation of residual fractions and has good stability with time on stream, its utility is remarkably improved when this catalyst is employed in a particular manner in combination with a second catalyst having different critical characteristics. For convenience, a catalyst of the type described in U.S. Pat. No. 3,876,523 will be referred to herein as a first catalyst, it being understood that this first catalyst is to be situated upstream of the second catalyst having different characteristics.